Troubleshooting

Ensure that you do not have any filters denying traffic (DNS or HTTP) that is required for DynDNS updates.

Enable full logging and inspect the logs for issues. Use the command 'set system syslog file dyn-debug any any'.

Additional

I found that when using IPv6 and PPPoE additional entries can be generated that contain the string "EVENT Add" which incorrectly triggers the DynDNS update script. Updating DynDNS with IP addresses that have not changed is deemed abuse and as a result you can be temporarily denied/banned from using DynDNS. To resolve this the following configuration change is required.